iMac - G5 power issues

I work at a computer repair shop and someone brought in an iMac G5 with an interesting problem. Attempting to put it to sleep (either from the Apple Menu or with the power button) causes it to turn off. The power settings appear to be kosher. After zapping the PRAM, I can get it to sleep, but it either restarts or gets stuck in sleep mode and has to be unplugged.